1. Field of the Invention
The present invention relates to a motor drive control system which performs drive control of a motor and, more particularly, relates to a motor drive control system which is for switching rectangular wave energization and PWM energization depending on the drive condition of a motor.
2. Description of the Related Art
A motor drive control system (hereinafter, referred to as an “inverter”) is arranged in order to generate an alternating current (AC) voltage and an AC current for driving an electric motor (motor) by means of a DC power supply. The inverter is arranged with a plurality of switching devices, generates the AC voltage and the AC current by controlling an energization sequence of the switching devices, and controls the amplitude of the AC voltage and the AC current by controlling an energization time of the switching devices. An energization method of the switching devices generally includes pulse width modulation energization (hereinafter, referred to as “PWM energization”) and rectangular wave energization.
The PWM energization is the energization method which changes ON time of the switching devices to control the magnitude of output voltage and can easily control the output voltage; and therefore, the PWM energization has an advantage capable of stably rotating even at low rotation. However, the voltage amplitude capable of outputting by PWM energization is restricted; and accordingly, a problem exists that the motor cannot be driven at high rotation.
On the other hand, the rectangular wave energization is the energization method which makes the switching devices ON by a half cycle of one cycle in electrical angle and can increase the output voltage as compared to the PWM energization; and therefore, it becomes possible to make the motor drive at high rotation. Further, switching of the switching devices is performed only two times at one cycle in electrical angle; and therefore, the rectangular wave energization has an advantage in which the number of switching of the switching devices is smaller than that of the PWM energization and switching loss is reduced to be able to increase inverter efficiency. However, ON time of the switching devices cannot be freely changed; and accordingly, a problem exists that control of the output voltage is difficult.
For example, JP2005-218299A proposes a technique in which an actual current amplitude and an actual current phase to be energized to the motor are detected by a current sensor; a voltage amplitude and a voltage phase are calculated by using the detected value; a selection is made as to which control of the rectangular wave energization or the PWM energization will be performed to drive the motor in consideration of a torque command value, a direct current (DC) voltage, and the calculated voltage amplitude and voltage phase; and the motor is driven efficiently from low speed to high speed.
However, according to the technique disclosed in Patent Document 1, the current sensor which is for detecting the AC current to be energized to the motor has to be arranged in a harness connected between the motor and the inverter. Accordingly, motor output is lowered due to electric power loss of the harness. Furthermore, if the current sensor is brought down, switching determination of the PWM energization and the rectangular wave energization cannot be performed; and accordingly, a problem exists in that the motor cannot be stably driven during the breakdown of the current sensor.